Pluto's discovery was kept quiet for 23 days in 1930

Lowell Observatory announced the discovery of Pluto on March 13, 1930, 23 days after Clyde Tombaugh first spotted it on February 18. The news went out through Harvard College Observatory, at Lowell’s request.

The delay was caution. Astronomers had once before believed they had found Planet X, only for it to turn out to be an asteroid, so the observatory kept quiet for weeks while it took more images and measurements. An unnamed staff member later wrote that Lowell had been criticised for not sharing the news sooner, and defended the decision to check first.

From search to name

• 1905: Percival Lowell started the search for a ninth planet.
• February 18, 1930: Tombaugh first spotted the object.
• February 23: a staff letter to Roger Putnam said more observations were needed before anyone could be sure.
• March 12: Putnam telegraphed Harvard with the object’s position and brightness.
• March 13: Harvard released the news on an announcement card giving its location and possible orbit. The date was Lowell’s birthday and the anniversary of William Herschel’s discovery of Uranus.
• March 14: at breakfast in Oxford, 11-year-old Venetia Burney heard the news from her grandfather and suggested “Pluto”. The name became official on May 1, 1930.

The announcement made one claim that did not last: it presented the object as the planet Lowell had predicted. To explain the orbital irregularities Lowell had calculated, his planet needed to be at least roughly as massive as Earth. The new world showed up only as a point of light that large telescopes could not resolve into a disc, so it was clearly small.

Nobody knew it yet, but Tombaugh had found the first known member of the Kuiper Belt, the crowded region that led to Pluto’s reclassification in 2006.